/**
 * 把url转换为 canvas对象
 * @param url 网络图片地址必须服务器设置允许跨域
 * @returns {Promise<any>}
 */
export const urlToCanvas = (img) => {
  function getBase64Image(img,width,height) {//width、height调用时传入具体像素值，控制大小 ,不传则默认图像大小
    var canvas = document.createElement("canvas");
    canvas.width = width ? width : img.width;
    canvas.height = height ? height : img.height;

    var ctx = canvas.getContext("2d");
    ctx.drawImage(img, 0, 0, canvas.width, canvas.height);
    var dataURL = canvas.toDataURL();
    return dataURL;
}
var image = new Image();
image.crossOrigin = '';
image.src = img;
var deferred=$.Deferred();
if(img){
    image.onload =function (){
        deferred.resolve(getBase64Image(image));//将base64传给done上传处理
    }
    return deferred.promise();//问题要让onload完成后再return sessionStorage['imgTest']
}
}